Statolith

Statolith

These specialized amyloplasts are denser than the cytoplasm and can sediment according to the gravity vector. They are found in a special subset of cells of the root cap (a tissue at the tip of the root) called statocytes. Statoliths are enmeshed in a web of actin and it is thought that their sedimentation transmits the gravitropic signal by activating mechanosensing channels. The gravitropic signal then leads to reorientation of auxinefflux carriers and subsequent redistribution of auxin streams in root cap and root as a whole. The changed relations in concentration of auxin leads to differerential growth of the root tissues. Taken together, the root is then turning, following the gravity stimuli.
Statocyts are also found in the endodermic layer of the inflorescence stem. The redistribution of auxin causes the shoot to turn in a direction opposite that of the gravity stimuli. As these are a subclass of amyloplasts, they originally come from leucoplasts [missing reference !, this statement is in inconsistency with shown figure and with Buchanan et al (eds) Biochemistry and Molecular Biology of Plants; Rockville, Maryland] . All of these different 'plasts' come from proplastids.